1. Field of the Invention
The present invention relates to a semiconductor device and more particularly to a semiconductor device having a semiconductor chip made of silicon carbide or gallium nitride mounted on a substrate for improvement of the connecting conduction of chip electrodes, which can thus be operated at a higher temperature.
2. Description of the Related Art
Semiconductor devices such as diodes, transistors, semiconductor lasers, and integrated circuits are widely utilized in a variety of industries. A most common type of the semiconductor devices is based on a single semi-conductive element such as general-purpose silicon (Si).
Such a known semiconductor device will now be explained as an example referring to a conventional discrete MOSFET which is generally used in a household apparatus or a switching source.
A MOSFET chip has a source electrode and a gate electrode provided on a front side of a substrate thereof made of a semiconductor material such as single element silicon, and has a drain electrode provided on the back side of the substrate. The drain electrode is soldered by a tin based soldering material to a metal frame, and then the source electrode and the gate electrode are electrically connected to an external source terminal and an external gate terminal, respectively, through metal wires which are connected by ultrasonic welding and the like. The MOSFET chip is then encapsulated in a molding resin such as epoxy resin which can be cured at a temperature lower than the melting point of a soldering material, so that the surfaces of MOSFET chip and the metal wires can not be exposed. The molding resin is then heated and cured.
As mentioned above, the general-use of a semi-conductive element such as silicon of a single element is known in the prior art. However, the use of a single semi-conductive element may be unfavorable in thermal and chemical stability, mechanical strength, and environmental durability.
Recently, the single semi-conductive element such as silicon (Si) has hence been replaced by other compound semiconductors which are favorably high in the environmental durability, and now ready for practical use, including such as silicon carbide (SiC), gallium arsenide (GaAs), and gallium nitride (GaN).
For example, as for the semiconductor technology with silicon carbide in relation to an environmental temperature and Schottky properties, a technology of the silicon carbide semiconductor is disclosed, in which the thermal stability can be increased and the semiconductor device can thus be operated in a higher temperature (for example, referred to Patent document 1: Japanese Patent Laid-open Publication 2000-106444).
Also, another modification is disclosed where the semiconductor of silicon carbide is increased in thermal stability and can be used at a higher temperature as being applicable to a large electric power device (for example, referred to Patent document 2: Japanese Patent Laid-open Publication (Showa) 64-65870).
The compound semiconductors such as silicon carbide and gallium nitride are greater in energy gap between bands than the single semiconductor such as silicon and can thus be improved in the thermal stability. Accordingly, semiconductor devices using silicon carbide or gallium nitride can be operated at a high temperature up to 1000 Kelvin, and is advantageously arranged with a higher density.
Moreover, the silicon carbide or gallium nitride semiconductor is substantially ten times greater in strength of a breakdown electric field and can thus minimize the width of a depletion layer for inhibiting a given level of voltage. Accordingly, in the inhibiting state of a given level of voltage, the semiconductor device arranged operable under a particular condition where a high voltage is generated may be made preferably of a compound semiconductor such as silicon carbide or gallium nitride. In other words, the semiconductor device of silicon carbide or gallium nitride can significantly be reduced in thickness than any single silicon semiconductor device for holding the voltage at a given level.
Thus, as the distance between the cathode and the anode can be reduced, the voltage drop substantially proportional to the electrode distance can be minimized during a flow of current. More particularly, the normal loss derived from the voltage drop during the current flow will be minimized. As the result, a diode or switching device based on silicon carbide or silicon nitride can remain lower in both the normal loss and the conversion loss during the switching action than any single silicon based semiconductor device. Also, the silicon carbide device is operable at a higher temperature than that of any single silicon based device, and therefore its cooling mechanism specifically such as a heat-sink and the like can hence be simplified when it acts as a power switching device.
The conventional sealing material for encapsulating a semiconductor chip is commonly an organic resin such as epoxy resin which may be decomposed when its temperature reaches higher than about 200° C. Its ionized particles will hence adhere to the semiconductor chip, which is thus declined in performance. Since the temperature of the sealing material such as epoxy resin is limited, the thermal stability pertinent to the semiconductor chip can not be implemented at its maximum level.
Whereas, an usable temperature limit of 170° C. in case of using the conventional epoxy resin as a sealing material for encapsulating a semiconductor chip and its electrodes can be increased to a temperature range from 350° C. to 450° C. when glass material is used instead of the epoxy resin (for example, referred to Patent document 3: Japanese Patent Laid-open Publication 62-205635).
Thus, when the compound semiconductor materials such as silicon carbide or gallium nitride are used in place of the known single semiconductor materials, the conventional semiconductor devices are successfully improved in antinomy relationship between the conversion loss and the normal loss during the switching action, unlike the silicon single material devices. However, the conventional semiconductor devices fail to overcome the problem of increase in a heat release value in a device which controls a great current flow.
Also, in the conventional semiconductor device, as the semiconductor chip electrodes are connected to external electrode terminals by soldering, its production procedure will be less simple and thus unfavorable in view of the productivity and production cost.
It is essential to secure the connections between the electrodes of the semiconductor chip and the corresponding external electrode terminals for allowing no disconnection of the semiconductor chip from the external electrode terminals. The greater the current control, the higher the heat energy will be increased. This requires the connections between the electrodes of the semiconductor chip and the corresponding external electrode terminals to have a higher level of heat radiation performance.
If the current flowing through the metal wires is increased, it may generate undesired heat or cause a voltage drop across the metal wires. In the case where the semiconductor device is implemented in a large power module form, its wiring arrangement increases in size and number and will take more time for installation, thus declining the production efficiency.
To achieve an inherent advantage that a semiconductor chip made of a compound semiconductor material such as silicon carbide or gallium nitride is operable in a high-temperature, it is hence desired for the sealing material for encapsulating the semiconductor chip and its electrodes to improve in heat resistance. The connection part between an external electrode on the surface of the semiconductor chip and the corresponding external electrode terminals are also required to be of a configuration having a higher heat resistance and a high-efficiency in radiation of the heat.